1. Field of the Invention
This invention relates to high porosity glass articles, particularly microporous glass beads, having high crush strength and high thermal stability. This invention also relates to methods for making, as well as to glass compositions particularly suited to the methods of making such articles. In a particular aspect, this invention relates to a method of improving even further the crush strength and the dimensional stability of the porous glass articles at elevated temperatures.
2. Brief Description of the Prior Art
It is well known in the art to make a porous glass article by heat treating the glass to cause phase separation and then leaching the glass to extract the soluble phase. Such a procedure is disclosed in U.S. Pat. Nos. 2,106,744 and 2,221,709, both to Hood et al., which describe a method of making porous glass from an alkali-borosilicate glass by heat treatment of the glass to separate it into two phases, one of which is an insoluble phase being rich in silica, and the other is a soluble phase being rich in boric oxide and in alkali. The borate-rich phase is soluble in acid and may be leached out leaving the insoluble phase as a rigid cellular structure which maintains the shape of the initial glass. These porous silica structures have been suggested for use as catalyst supports. Other references which disclose the use of various porous glasses for use as catalyst supports are U.S. Pat. Nos. 2,461,841 to Nordberg; 2,480,672 to Plank; 2,834,738 to Vincent and 2,835,367 to Dowden et al. Unfortunately, this utility is somewhat limited by the fact that porous glass structures up until the time of this invention have not been made with high pore volumes, mechanical strengths and thermal stabilities. For high catalytic activity, high pore volumes are desirable. For applications such as fluid catalyst beds and motor vehicle catalytic mufflers, poor mechanical strength and shrinkage of the catalyst support due to high temperature exposure is undesirable.